1. Field of the Invention
The present invention relates to an EL (electro luminescent) element drive circuit and an organic EL display device using the same drive circuit. In particular, the present invention relates to an improvement of an organic EL element drive circuit having a current drive circuit for current-driving organic EL elements connected to a column line, which is an anode side drive line of the organic EL elements of an organic EL display panel through terminal pins thereof and a small size additional circuit for improving power consumption of the current drive circuit, circuit size of which is small enough to restrict increase of the organic EL element drive circuit when the current drive circuit and the additional circuit are formed in an IC chip.
2. Description of the Prior Art
An organic EL display panel of an organic EL display device, which is mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance) and includes 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines, has been proposed and the number of column lines and the number of row lines of such organic EL display panel tend to be further increased.
An output stage of a current drive circuit of such organic EL display panel includes an output -circuit constructed with, for example, current-mirror circuits, which are provided correspondingly to respective terminal pins of the panel, regardless of the type of drive current, the passive matrix type or the active matrix type.
Incidentally, JPH9-232074A discloses a drive circuit for organic EL elements, in which the organic EL elements arranged in a matrix are current-driven and a terminal voltage of each organic EL element is reset by grounding an anode and a cathode of the organic EL element Further, JP2001-143867A discloses a technique with which power consumption of an organic EL display device is reduced by current-driving organic EL elements with using DC—DC converters.
Incidentally, in order to emphasize a display of telephone numbers, etc., on a portable telephone set, etc. it is usual to limit a display area for such information to a center portion of a display screen and to surround the display area by a black frame or another color background. Further, a background of the display screen may be displayed by one of three primary colors, red (R), green (G) and blue (B) for purpose of some warning display.
In the case of the one color display or the black background, organic EL elements for only one color or all of the R, G and B display colors are not actuated.
In the case of the black background or the partial actuation of the organic EL elements, it may be considered that, in order to reduce power consumption, switch circuits for stopping operations of output stage current sources are provided correspondingly to respective terminal pins of an organic EL display panel as additional circuits. In such case, however, there are problems that such additional circuits may increase the circuit size of an organic EL drive circuit and that a control of such additional circuits may become complicated.
Therefore, it has been usual that, in the case of the black frame background or the case where organic EL elements for any one of R, G and B display colors are not actuated, display data for preventing drive currents of these organic EL elements from generating is set in drive stages of the organic EL element drive circuit and the power consumption is reduced by stopping the drive current supply from the output stage of the organic EL element drive circuit to the terminal pins of the organic EL panel. In such case, the drive stage in which the display data is set must be operated even during the black frame background or the non-actuation period of the organic EL elements and, therefore, power consumption of the output stage for the organic EL elements for one horizontal display line corresponding to one horizontal scan direction can not be neglected.
On the other hand, the number of driving terminal pins of an organic EL panel tends to increase according to high resolution request. According to this tendency, the number of output stages of the current drive circuit tends to be increased with increase of the number of the driving terminal pins. Therefore, the circuit size of the organic EL element drive circuit is increased, so that the power consumption of the organic EL element drive circuit is increased.